Ultra low power multistage current mode logarithmic amplifier for fully implantable cochlear implants

Aslantürk, Cansu
Cochlear implants (CI) are used for severe to profound hearing losses. Since CIs suffer from damage risk, frequent battery replacement and aesthetic concern, Fully Implantable Cochlear Implants (FICI) are preferred. Due to limitations of FICI sensors, a front end circuitry that can process very low inputs is needed. In this thesis, multistage current mode logarithmic amplifiers (LA) are presented to be used as dynamic range compression and amplification blocks for FICI. MOSFET’s subthreshold region operation is combined with parallel amplification parallel summation (PAPS) method so that accurate and process invariant logarithmic behavior can be achieved with wide input dynamic range coverage and ultra-low power dissipation for signals in the nano-micro ampere range. Parasitic extracted (PEX) simulations show that PAPS circuit successfully compresses 40dB input dynamic range to 11.5dB output dynamic range with 2.5uW power dissipation for 20nA-2uA input currents. Channel selective current mode logarithmic amplifier aims to cover daily acoustic band; therefore, channel selection mechanism is implemented which pre-amplifies inputs when they are below certain threshold. This guarantees functionality of sub-circuits while increase in power dissipation is minimized. PEX simulation results show that channel selective LA compresses 52dB dynamic range to 13dB for 5nA-2uA input currents while dissipating 19.17uW. Both PAPS and channel selective LAs have two versions: externally biased and self biased. Externally biased versions can be used with various sensors while self biased versions eliminate dependency to external source. Presented logarithmic amplifiers are also combined with remaining FICI interface circuit blocks and system compatibility is confirmed.
Citation Formats
C. Aslantürk, “Ultra low power multistage current mode logarithmic amplifier for fully implantable cochlear implants,” M.S. - Master of Science, Middle East Technical University, 2024.